The present invention relates to a piezoelectric actuator which is utilized to precisely move tools for a precision machine tool, pistons for a hydraulically controlled apparatus, samples in an electron microscope, or the like in the order of microns.
A conventional actuator having an electrostrictive element assembly is shown in FIG. 1 of the accompanying drawing. It comprises a laminated body of a plurality of sheet-like electrostrictive elements 1 which are connected to each other mechanically in series and electrically in parallel, and are axially displaced when a voltage is applied thereto.
Each of the electrostrictive elements 1 is provided with electrode layers 1a and 1b on the major surfaces which are connected every other. It is arranged so that the electrode layer 1a on one major surface of each electrostrictive element 1 has one polarity and the electrode layer 1b on the other major surface of each electrostrictive element 1 another polarity. The electrostrictive elements 1 are laminated in such a manner that the electrode layers 1a and 1a, or lb and 1b having the same polarity of the adjacent electrostrictive elements are abutted or connected to each other. The exposed end portions of the abutted electrode layers 1a and 1a or 1b and 1b between the adjacent electrostrictive elements 1 are alternately coated with insulating material 2 so that the electrode layers 1a having one polarity are electrically separated from those layers 1b having the other polarity. The electrode layers 1a having one polarity are connected to one electrode terminal 3a of a pair of electrode terminals 3a and 3b which are provided on the side edge portions of the laminated body and the electrode layers 1b having the other porality are connected to the other electrode terminal 3b.
Such actuators are also disclosed in U.S. Pat. Nos. 4,523,121 and 4,011,474.
Furthermore, in U.S. Pat. No. 4,523,121 there is disclosed another arrangement which comprises a plurality of electrostrictive elements defined by a plurality of internal electrodes, each internal electrode has one end terminated on a lateral edge portion of the associated element and the other end of each internal electrode is terminated adjacent to the opposite lateral edge portion of the element to prevent the other end from making contact with a pair of external electrodes which are provided along the lateral edges of the electrostrictive elements.
A similar arrangement is also disclosed in U.S. Pat. No. 4,087,716, in which a plurality of laminations of piezoelectric ceramic material is provided, on the upper size of each layer and underneath each layer are provided metal layers which are alternately connected to external connection contacts provided along the lateral edges of the laminations, and the metal layers overlapped between the adjacent ceramic layers are intended not to occupy the entire area of the ceramic layers in order to provide sufficient mutual electrical insulation of the oppositely poled metal layers at the edges of the ceramic layers.
With such a conventional laminated type piezoelectric actuator as illustrated in FIG. 1, when an insulating paint casting is used to cover the exposed end portions of the electrode layers between the adjacent electrostrictive elements, it is difficult to form the actuator with a certain precision in dimensions because some protrusion in the direction of the lamination may unavoidably be formed by the insulator coating. Such protrusion of the insulator coatings may be rubbed and exfoliated upon assembling of the actuator into a desired apparatus and therefore the outer edge of the electrode layers may be exposed causing a deterioration of the insulating property.
Further, in the conventional arrangement as disclosed in U.S. Pat. No. 4,523,121 or 4,087,716, the above mentioned disadvantages do not occur, but there may be produced a distortion at the abutted or superimposed portions having the same polarity between the adjacent electrostrictive elements While there may be produced no distortion at the portions, insulating gaps, not superimposed between the adjacent electrostrictive elements when the voltage is applied. Therefore, a crack may be produced in the insulating gaps by means of an internal stress when the electric field is increased beyond a certain level.